Chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been adopted as techniques for depositing thin films to comply with the scaling down of semiconductor devices. The film growth usually results from the chemical reaction of one or more precursor compounds, so it is essential to develop optimum precursors and understand their reaction process. Sometimes a reactant is introduced to react with the precursor(s), so the choice of an appropriate reactant is also of high importance, as it impacts the film's properties as directly as the precursors. Finally, the deposition process is also important. In some cases the use of a plasma source is necessary to tune the film properties. However, plasma may damage the sub-layers and may not allow deposition of films with a sufficient conformality, especially in deep trenches or holes typically used by memory makers.
Ruthenium (Ru) is a potential candidate for thin film applications in the industrial semiconductor manufacturing process for many applications in the coming years. For the next generation nodes, Ru is considered as one of the best candidates for the electrode capacitor for FeRAM and DRAM applications. Ru has interesting properties, such as a high melting point, low resistivity, high oxidation resistance and adequate work functions, making it also a potential gate electrode material for CMOS transistors.
In view of the aforementioned properties of ruthenium oxide, ruthenium oxide-based materials, such as CaRuO3, SrRuO3 and BaRuO3, may be used as an electrode for ferroelectric and DRAM applications. Strontium ruthenium oxide, SrRuO3 (SRO) films are of interest because they exhibit a perovskite crystal structure whose lattice mismatch with strontium titanium oxide, SrTiO3 (STO) is very small (3.93 Å vs. 3.91 Å). CaRuO3 films are of interest because they exhibit a lower resistivity than SRO. Hence, it is expected that ARuO3 (wherein A=Sr, Ca or Ba) films may be suitable to be used as a conductive layer for gate or electrode applications.
WO2009/118708 discloses methods and compositions for the deposition of ternary oxide films containing ruthenium and an alkali earth metal. In its examples, WO2009/118708 discloses SRO film deposition using H2O as a reactant in a CVD process and using a mixture of H2O and oxygen in an ALD process. U.S. Pat. No. 7,544,389 discloses ALD and CVD methods for the deposition of ruthenium and ruthenate films using a solution of ruthenium tetroxide. Specifically, U.S. Pat. No. 7,544,389 discloses Ru film deposition using the RuO4 precursor and H2 as a reactant, RuO2 film deposition using the RuO4 precursor and a heated substrate, and ruthenate film deposition using the RuO4 precursor, an organometallic precursor, and an oxygenated gas, such as O2, O3, or N2O.
When SRO films are deposited, especially using ALD, the fit between strontium and ruthenium precursors is of extreme importance. For example, most of the strontium precursors lead to strontium oxide (SrO2) films when ozone (O3) or moisture (H2O) is used as a reactant, but not oxygen (O2). However, ozone is generally known to be able to react with ruthenium even at 20° C., generating volatile RuO4. Such equilibrium between deposition and “etching” if ozone is used can not be tolerated.
Consequently, there is a need for developing an ALD process that enables the deposition of thin films of SrRuO3 (or CaRuO3 and BaRuO3) having a good uniformity even in most advanced device geometries (aspect ratio of 20:1 or higher) at lower temperatures than the current methods (in other words, at temperatures lower than 350° C.).